Solid electrolytic capacitors with improved reliability

ABSTRACT

A capacitor with an anode, a dielectric on the anode and a cathode on the dielectric. A blocking layer is on the cathode. A metal filled layer is on said blocking layer and a plated layer is on the metal filled layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation-in-part of U.S. patentapplication Ser. No. 12/469,786 filed May 21, 2009 now U.S. Pat. No.8,310,816 which is incorporated herein by reference

BACKGROUND

The present invention is related to an improved method of forming asolid electrolyte capacitor and an improved capacitor formed thereby.More specifically, the present invention is related to an improvedmethod of electrically connecting a cathode to a cathode lead in acapacitor and an improved capacitor formed thereby.

The construction and manufacture of solid electrolyte capacitors is welldocumented. In the construction of a solid electrolytic capacitor avalve metal serves as the anode. The anode body can be either a porouspellet, formed by pressing and sintering a high purity powder, or a foilwhich is etched to provide an increased anode surface area. An oxide ofthe valve metal is electrolytically formed to cover all surfaces of theanode and serves as the dielectric of the capacitor. The solid cathodeelectrolyte is typically chosen from a very limited class of materials,to include manganese dioxide or electrically conductive organicmaterials such as 7,7′,8,8′-tetracyanoquinonedimethane (TCNQ) complexsalt, or intrinsically conductive polymers, such as polyaniline,polypyrol, polythiophene and their derivatives. The solid cathodeelectrolyte is applied so that it covers all dielectric surfaces. Animportant feature of the solid cathode electrolyte is that it can bemade more resistive by exposure to high temperatures. This featureallows the capacitor to heal leakage sites by Joule heating. In additionto the solid electrolyte the cathode of a solid electrolyte capacitortypically consists of several layers which are external to the anodebody. In the case of surface mount constructions these layers typicallyinclude: a carbon layer; a metal filled layer containing a highlyconductive metal, typically silver, bound in a polymer or resin matrix;a conductive adhesive layer such as silver filled adhesive; and a highlyconductive metal lead frame. The various layers connect the solidelectrolyte to the outside circuit and also serve to protect thedielectric from thermo-mechanical damage that may occur duringsubsequent processing, board mounting, or customer use.

In the case of conductive polymer cathodes the conductive polymer istypically applied by either chemical oxidation polymerization,electrochemical oxidation polymerization or spray techniques with otherless desirable techniques being reported.

The carbon layer serves as a chemical barrier between the solidelectrolyte and the metal filled layer. Critical properties of the layerinclude adhesion to the underlying layer, wetting of the underlyinglayer, uniform coverage, penetration into the underlying layer, bulkconductivity, interfacial resistance, compatibility with the silverfilled layer, buildup, and mechanical properties.

The silver filled layer serves to conduct current from the lead frame tothe cathode and around the cathode to the sides not directly connectedto the lead frame. The critical characteristics of this layer are highconductivity, adhesive strength to the carbon layer, wetting of thecarbon layer, and acceptable mechanical properties. Compatibility withthe subsequent layers employed in the assembly and encapsulation of thecapacitor are also critical. In the case where a silver filled adhesiveis used to attach to a lead frame compatibility between the lead frameand the silver filled adhesive is necessary. In capacitors which utilizesolder to connect to the external lead, solderability and thermalstability are important factors. In order for the solder to wet themetal filled layer, the resin in the metal filled layer must degradebelow the temperature at which the solder is applied. However, excessivedegradation of the resin creates an effect termed “silver leeching”resulting in a poor connection between the external cathode layers andthe external cathode lead. The traditional approach to applying a silverfilled layer requires a delicate compromise in thermal stability of theresin in order to simultaneously achieve solder wetting and to avoidsilver leeching. The silver filled layer is secured to a cathode leadframe by an adhesive. The adhesive is typically a silver filled resinwhich is cured after the capacitor is assembled.

Reliability of the capacitors requires that the interface between thesilver filled layer and carbon layer, and the interface between thesilver filled layer and adhesive layer, have good mechanical integrityduring thermo mechanical stresses. Solid electrolytic capacitors aresubject to various thermomechanical stresses during assembly, molding,board mount reflow etc. A weak interface with the silver filled layercan cause delamination of the layers which causes reliability issues.Solid electrolytic capacitors are also required to have goodenvironmental properties such as good chemical and moisture resistance.Reliability issues caused by silver migration under humid conditions areknown in the electronics industry. Silver metal from the silver filledlayer can migrate to the anode causing high leakage current.

U.S. Pat. Nos. 4,000,046, and 4,104,704 teach an electroplating methodfor solid electrolytic capacitors. Electroplating was performed on waterbased graphite coatings and silver paint coatings. Experiments toreproduce the method of this disclosure showed significant reliabilityissues such as high leakage current and electrical shorts.Investigations to understand this suggest that the diffusion of theplating electrolyte through this hydrophilic and porous conductive layerto the semi conductive layer and anode is influencing the reliability.It is also found that the top of the anode with no carbon layer providessignificantly more permeability for the plating electrolyte diffusion.

Silver filled coatings are used in solid electrolytic capacitors forcurrent collection from the cathode. Highly conductive silver filledcoatings enable lower ESR compared to other metal particle filledcoatings. However, the capacitors using these polymeric cathode coatingssystems suffer from ESR shift on exposure to Surface Mount Technology(SMT) conditions. During board mount the capacitors are subjected toelevated temperatures which create stresses in the interfaces due tocoefficient of thermal expansion (CTE) mismatches. This stress causesdelamination and thus an increased ESR in the finished capacitor.

There has been an ongoing desire for a capacitor which has a highconductivity layer, for low ESR, which can be surface mounted withoutdetriment to the ESR. The present invention provides such a capacitor.

SUMMARY

It is an object of the present invention to provide a solid electrolyticcapacitor with improved reliability by improving the layers between thecathode and lead frame.

A particular feature of the improved cathode is the improvedreliability.

Another advantage is the low ESR which can be achieved and a decrease inthe ESR shift which typically occurs upon surface mounting.

These and other advantages, as will be realized, are provided in animproved a capacitor. The capacitor has an anode, a dielectric on theanode and a cathode on the dielectric. The capacitor also has a platedmetal layer a blocking layer between the cathode and plated metal layer.

Yet another embodiment is provided in a method for forming a capacitor.The method includes the steps of:

providing an anode;

forming a dielectric on the anode;

applying a cathode on the dielectric;

applying a blocking layer; and

plating a metal layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional schematic view of a prior art capacitor.

FIG. 2 is a cross-sectional schematic view of an embodiment of theinvention.

FIGS. 3A-3C are a partial cross-sectional schematic views of embodimentsof the present invention.

FIG. 4 is a schematic illustration of an embodiment of the presentinvention.

FIG. 5 is a partial cross-sectional view of an embodiment of the presentinvention.

FIG. 6 is a partial cross-sectional view of an embodiment of the presentinvention.

DETAILED DESCRIPTION

The present invention mitigates the deficiencies of the prior art byproviding a blocking layer and plated metal layer between the cathodeand the lead frame. The blocking layer increases productivity withoutdetriment to the electrical properties of the capacitor. The presentinvention will be described with reference to the various figures whichillustrate, without limiting, the invention. Throughout the descriptionsimilar elements will be numbered accordingly.

By the method of this invention, the inventors discovered that ultra lowESR can be achieved by metal plating, preferably nickel plating, inconcert with a blocking layer. This method mitigates the ESR shifttypically observed with SMT and on exposure to humidity. Furthermore, anadditional protective layer, in addition to the blocking layer, furtherreduces ESR shift.

FIG. 1 illustrates a cross-sectional schematic view of a prior artcapacitor generally represented at 10. The capacitor comprises an anode,11, preferably comprising a valve metal as described further herein withan anode wire, 18, extending there from or attached thereto. Adielectric layer, 12, is provided on the surface of the anode, 11.Coated on the surface of the dielectric layer, 12, is a cathode layer,13. A carbon layer, 14, and metal filled layer, 16, provide electricalconductivity and provide a surface which is more readily adhered to thecathode terminal, 17, than is the cathode layer, 13. An adhesive layer,21, secures the cathode lead to the cathode terminal. The anode wire,18, is electrically connected to the anode terminal, 19, by a connector,23, which may be integral to a lead frame. The entire element, exceptfor the terminus of the terminals, is then preferably encased in anon-conducting material, 20, such as an epoxy resin.

An embodiment of the present invention is illustrated schematically inFIG. 2 at 50. In FIG. 2 the anode, 11; dielectric, 12; cathode, 13;cathode termination, 17; anode wire, 18; anode termination, 19;non-conducting material, 20; and connector, 23, are as illustratedrelative to FIG. 1. A metal filled layer, 31, preferably a silver filledlayer, is on the blocking layer, 30, and a metal layer, 40, is on themetal filled layer. The blocking layer comprises at least one layerselected from the group consisting of a hydrophobic layer, an insulativelayer and a carbon layer in a crosslinked matrix. The blocking layerinhibits migration of metals and metal ions towards the dielectric. In aparticularly preferred embodiment the blocking layer is between firstand second carbon layers. The blocking layer preferably encases theentire underlying structure. A second blocking layer, 30′, is preferablydisposed on at least a portion of the surface of the underlying monolithfrom which the anode wire, 18, extends. The second blocking layer may bethe same as the blocking layer. Alternatively, the second blocking layermay be a layer which is different from the blocking layer.

The function of the blocking layer is to inhibit metal and metal ionsfrom migrating there through without significant degradation inelectrical conduction therethrough. Each surface of the blocking layermust be compatible with the layer attached thereto. A metallic layer,40, on the metal filled layer provides improvements in ESR withoutdetrimental change when surface mounted.

An embodiment of the invention is illustrated in FIG. 3A wherein across-sectional portion with the cathode, 13, metallic layer, 40, andlayers there between shown in isolation. In the embodiment of FIG. 3A afirst carbon layer, 35, is in contact with the cathode and the layer isformulated to adhere adequately to the cathode while still havingadequate conductivity through the layer. The blocking layer, 32,inhibits the metal ion in the electroplating electrolyte from migratinginto or through the blocking layer. It is preferred that no metalmigrates through the blocking layer. In practice, minute amounts maymigrate which is undesirable but acceptable. A second carbon layer, 33,is formulated to provide adhesion to the blocking layer and to the metalfilled layer, 31. A metallic layer, 40, is on the metal filled layer.The metallic layer, 40, is the eventual contact point within a circuitand is electrically connected to a cathode lead or to a circuit tracepreferably by a conductive adhesive.

Another embodiment of the invention is illustrated in FIG. 3B wherein across-sectional portion with the cathode, 13, metallic layer, 40, andlayers there between shown in isolation. In FIG. 3B the blocking layer,32, is between the cathode, 13, and the carbon layer, 35. Thisembodiment has the advantage of requiring one less layer. A relatedembodiment is illustrated in FIG. 3C wherein the blocking layer, 32, isbetween the carbon layer, 35, and the metal filled layer, 31.

Another embodiment of the invention is illustrated in FIG. 6. In FIG. 6,a carbon layer, 35, is on the cathode, 13. Metal filled layers, 31,sandwich a blocking layer, 32, and a metallic layer, 40, is on theoutermost metal filled layer.

The cathode layer is a conductive layer preferably comprising conductivepolymer, such as polythiophene, polyaniline, polypyrrole or theirderivatives; manganese dioxide, lead oxide or combinations thereof. Anintrinsically conducting polymer is most preferred.

A particularly preferred conducting polymer is illustrated in Formula I:

R¹ and R² of Formula 1 are chosen to prohibit polymerization at theβ-site of the ring. It is most preferred that only α-site polymerizationbe allowed to proceed. Therefore, it is preferred that R¹ and R² are nothydrogen. More preferably, R¹ and R² are α-directors. Therefore, etherlinkages are preferable over alkyl linkages. It is most preferred thatthe groups are small to avoid steric interferences. For these reasons R¹and R² taken together as —O—(CH₂)₂—O— is most preferred.

In Formula 1, X is S or N and most preferable X is S.

R¹ and R² independently represent linear or branched C₁-C₁₆ alkyl orC₂-C₁₈ alkoxyalkyl; or are C₃-C₈ cycloalkyl, phenyl or benzyl which areunsubstituted or substituted by C₁-C₆ alkyl, C₁-C₆ alkoxy, halogen orOR³; or R¹ and R², taken together, are linear C₁-C₆ alkylene which isunsubstituted or substituted by C₁-C₆ alkyl, C₁-C₆ alkoxy, halogen,C₃-C₈ cycloalkyl, phenyl, benzyl, C₁-C₄ alkylphenyl, C₁-C₄ alkoxyphenyl,halophenyl, C₁-C₄ alkylbenzyl, C_(r) C₄ alkoxybenzyl or halobenzyl, 5-,6-, or 7-membered heterocyclic structure containing two oxygen elements.R³ preferably represents hydrogen, linear or branched C₁-C₁₆ alkyl orC₂-C₁₈ alkoxyalkyl; or are C₃-C₈ cycloalkyl, phenyl or benzyl which areunsubstituted or substituted by C₁-C₆ alkyl.

The conducting polymer is preferably chosen from polypyrroles,polyanilines, polythiophenes and polymers comprising repeating units ofFormula I, particularly in combination with organic sulfonates. Aparticularly preferred polymer is 3,4-polyethylene dioxythiophene(PEDT). The polymer can be applied by any technique commonly employed informing layers on a capacitor including dipping, spraying oxidizerdopant and monomer onto the pellet or foil, allowing the polymerizationto occur for a set time, and ending the polymerization with a wash. Thepolymer can also be applied by electrolytic deposition as well known inthe art.

The manganese dioxide layer is preferably obtained by immersing an anodeelement in an aqueous manganese nitrate solution. The manganese oxide isthen formed by thermally decomposing the nitrate at a temperature offrom 200 to 350° C. in a dry or steam atmosphere. The anode may betreated multiple times to insure optimum coverage.

As typically employed in the art, various dopants can be incorporatedinto the polymer during the polymerization process. Dopants can bederived from various acids or salts, including aromatic sulfonic acids,aromatic polysulfonic acids, organic sulfonic acids with hydroxy group,organic sulfonic acids with carboxylhydroxyl group, alicyclic sulfonicacids and benzoquinone sulfonic acids, benzene disulfonic acid,sulfosalicylic acid, sulfoisophthalic acid, camphorsulfonic acid,benzoquinone sulfonic acid, dodecylbenzenesulfonic acid, toluenesulfonicacid. Other suitable dopants include sulfoquinone,anthracenemonosulfonic acid, substituted naphthalenemonosulfonic acid,substituted benzenesulfonic acid or heterocyclic sulfonic acids asexemplified in U.S. Pat. No. 6,381,121 which is included herein byreference thereto.

Binders and cross-linkers can be also incorporated into the conductivepolymer layer if desired. Suitable materials include poly(vinylacetate), polycarbonate, poly(vinyl butyrate), polyacrylates,polymethacrylates, polystyrene, polyacrylonitrile, poly(vinyl chloride),polybutadiene, polyisoprene, polyethers, polyesters, silicones, andpyrrole/acrylate, vinylacetate/acrylate and ethylene/vinyl acetatecopolymers.

The first carbon layer and second carbon layer, which may be the same ordifferent, each comprises a conductive composition comprising at least5% resin by dry weight and conductive carbon particles. More preferablythe conductive composition comprises at least 20% resin by dry weight.Each carbon layer may individually also comprise adjuvants such ascrosslinking additives, surfactants and dispersing agents. The resin,conductive carbon particles and adjuvants are preferably dispersed in anorganic solvent or water to form a coating solution. The solvent andresin for the first conductive carbon layer needs to have goodwettability to the semi-conductive cathode surface.

The blocking layer comprises at least one layer selected from the groupconsisting of a hydrophobic layer, an insulating layer and a layercomprising carbon in a crosslinked matrix.

The blocking layer is most preferably less than two microns thick. Aboveabout two microns the resistivity of the layer exceeds acceptable limitsthereby defeating one of the purposes of the blocking layers. The lowerlimit of thickness is set by the coating technique with a monolayer onthe entire surface being the theoretical limit. This theoretical limitis difficult to reach with most coating techniques due to the presenceof surface vacancies wherein the blocking properties are compromised.Since the blocking layer may be a poorly conducting layer its presencemay increase resistance between the cathode and cathode lead which isundesirable.

The hydrophobic coating preferably comprises hydrophobic polymers.Silicone and their copolymers, fluorinated polymers and their copolymersare mentioned as being particularly preferred. The hydrophobic layer mayinclude fillers such as silica. Nanoclay and related materials modifiedwith a hydrophobic coating is particularly suitable for demonstration ofthe invention. The hydrophobic coating is preferably a thermoset coatingwith high cross link density. The hydrophobic coating is chosen suchthat the plating electrolyte has very low wettability to the coatedsurface. In addition to providing low wettability the high cross linkdensity prevents diffusion of plating electrolyte through this coatinglayer.

The layer comprising carbon in a crosslinked matrix comprises conductivecarbon particles with particularly preferred carbon particles selectedfrom graphite, carbon black, carbon nanotubes and graphene. The carbonis in a matrix of crosslinked resin wherein the preferred resins arepolymers of materials selected from the group phenolic, phenoxy, epoxy,acrylic, cellulose derivatives, aromatic cyanate esters, diallylisophthalate, bismaleimide, polyimides, polyamide imides, polysulfones,polyphylenes, polyether sulfones, polyaryl ethers, polyphenylenesulfides, polyarylene ether ketones, polyether imides, polyquinoxalines,polyquinolines, polybenzimidazoles, polybenzoxazoles,polybenzothiazoles, and silicones such as silicone polyester andsilicone epoxy. More preferably the resin is selected from cellulosederivatives, acrylic, polyester, aromatic cyanate ester, epoxy,phenolic, diallyl isophthalate, phenoxy, polyimide and bismaleimide. Theresin is preferably chemically crosslinked.

A second carbon layer is preferably applied over the blocking layer.Since the blocking layer is designed to have low wettability to aqueousbased systems, a water based carbon coating has very low adhesion tothis surface. A solvent based carbon coating is preferred for thisapplication. The solvent and resin of the carbon coating is chosen suchthat the coating can adequately wet the blocking layer which istypically a hydrophobic surface. In addition to wetting, the binder ofthe second carbon coating needs to have strong adhesion to the binder inthe blocking layer as well as to the metal filled layer. In addition tothe carbon particles such as graphite, carbon black, carbon nanotubes,graphene, metal particles can also be added to improve conductivity.

Preferred resins for the carbon layers are polymers of materialsselected from the group phenolic, phenoxy, epoxy, acrylic, cellulosederivatives, aromatic cyanate esters, diallyl isophthalate,bismaleimide, polyimides, polyamide imides, polysulfones, polyphylenes,polyether sulfones, polyaryl ethers, polyphenylene sulfides, polyaryleneether ketones, polyether imides, polyquinoxalines, polyquinolines,polybenzimidazoles, polybenzoxazoles, polybenzothiazoles, and siliconessuch as silicone polyester and silicone epoxy. More preferably the resinis selected from cellulose derivatives, acrylic, polyester, aromaticcyanate ester, epoxy, phenolic, diallyl isophthalate, phenoxy, polyimideand bismaleimide.

The plated metal layer is preferably applied to the metal filled layer.Plating can be done with various metallic systems. Nickel is a preferredmetal system. Plating can be done either by electroplating orelectroless plating. Electroplating is preferred due to the lowerproduction cycle time. Conductive adhesive is typically used toadhesively attach the metal layer to the lead frame which acts as thecathode lead or to a circuit trace. The thickness of the plated metallayer is preferably at least 2 microns to no more than 100 microns.Below about 2 microns there may not be complete coverage of thecapacitor with nickel due to surface roughness of the underlyingcathode. Above about 100 microns there is no further advantage offeredand any additional material increases material cost and processing time.

A preferred process for forming the capacitor is illustrated in FIG. 4.

In FIG. 4, the anode is formed, 100, preferably from a valve metal asdescribed further herein.

The anode is a conductor preferably selected from a valve metal or aconductive metal oxide. More preferably the anode comprises a valvemetal, a mixture, alloy or conductive oxide of a valve metal preferablyselected from Al, W, Ta, Nb, Ti, Zr and Hf. Most preferably the anodecomprises at least one material selected from the group consisting ofAl, Ta, Nb and NbO. Conductive polymeric materials may be employed as ananode material. Particularly preferred conductive polymers includepolypyrrole, polyaniline and polythiophene. Aluminum is typicallyemployed as a foil while tantalum is typically prepared by pressingtantalum powder and sintering to form a compact. For convenience inhandling, the valve metal is typically attached to a carrier therebyallowing large numbers of elements to be processed at the same time.

The anode is preferably etched to increase the surface area,particularly, if the anode is a valve metal foil such as aluminum foil.Etching is preferably done by immersing the anode into at least oneetching bath. Various etching baths are taught in the art and the methodused for etching the anode is not limited herein.

The anode wire is preferably attached to the anode, particularly when acompact is employed. The anode wire can be attached by welding or byembedding into the powder prior to pressing. A valve metal is aparticularly suitable anode wire and in a preferred embodiment the anodeand anode wire are the same material.

A dielectric is formed, 101, on the surface of the anode. The dielectricis a non-conductive layer which is not particularly limited herein. Thedielectric may be a metal oxide or a ceramic material. A particularlypreferred dielectric is the oxide of a metal anode due to the simplicityof formation and ease of use. The dielectric layer is preferably anoxide of the valve metal as further described herein. It is mostdesirable that the dielectric layer be an oxide of the anode. Thedielectric is preferably formed by dipping the anode into an electrolytesolution and applying a positive voltage to the anode. Electrolytes forthe oxide formation are not particularly limiting herein but exemplarymaterials can include ethylene glycol; polyethylene glycol dimethylether as described in U.S. Pat. No. 5,716,511; alkanolamines andphosphoric acid, as described in U.S. Pat. No. 6,480,371; polar aproticsolvent solutions of phosphoric acid as described in U.K. Pat. No. GB2,168,383 and U.S. Pat. No. 5,185,075; complexes of polar aproticsolvents with protonated amines as described in U.S. Pat. No. 4,812,951or the like. Electrolytes for formation of the dielectric on the anodeincluding aqueous solutions of dicarboxylic acids, such as ammoniumadipate are also known. Other materials may be incorporated into thedielectric such as phosphates, citrates, etc. to impart thermalstability or chemical or hydration resistance to the dielectric layer.

A conductive layer is formed, 102, on the surface of the dielectric. Theconductive layer acts as the cathode of the capacitor. The cathode is aconductor preferably comprising at least one conductive materialselected from manganese dioxide and a conductive polymeric material.Particularly preferred conductive polymers include polypyrrole,polyaniline and polythiophene. Metals can be employed as a cathodematerial with valve metals being less preferred.

After conductive cathode layer formation, 102, the layers between thecathode and metal plated layer are formed, 103. At least one blockinglayer is applied, 105, by any one or combination of the methods selectedfrom spraying, dipping, brushing, printing, and ink jet. It ispreferably that at least one carbon layer is applied, 104, and at leastone silver filled layer, 106, is applied both preferably by spraying ordipping. The blocking layer can be on either side of a carbon filledlayer or on either side of a metal filled layer.

A metal plated layer is formed at 108, preferably, onto a metal filledlayer and preferably by electroplating or electroless plating. In apreferred embodiment the metal plated layer is formed by reverse biaswherein the positive electroplating electrode is electrically connectedto the capacitors cathode and the negative electroplating electrode iselectrically connected to the anode lead.

The capacitor may be a discrete capacitor or an embedded capacitor. If adiscrete capacitor is to be formed, at 109, a conductive adhesive isadded, 110, and the metal layer is adhered to a cathode lead, 111. Thecapacitor is finished, 112, which may include incorporating anode andcathode terminals, external insulation, testing, packing and the like asknown in the art.

If the capacitors are to be employed in an embedded application orattached directly to a circuit trace the capacitors are finished, 113,which may include testing, packing and the like.

An embodiment of the invention is illustrated in partial cross-sectionalview in FIG. 5. In FIG. 5, a cathode, 201, comprises a blocking layer,202, between the cathode, 201, and a metal filled layer, 203.

The capacitor is illustrated herein as a discrete capacitor forconvenience and this is a preferred embodiment. In another preferredembodiment the anode wire and metallic layer may be in direct electricalcontact with a circuit trace wherein elements of the circuit mayconstitute the cathode lead, anode lead or both. In another preferredembodiment the capacitor may be embedded in a substrate or incorporatedinto an electrical component with additional functionality.

A metallic layer on the silver filled polymer cathode coating layeroffers a number of advantages. There is an ESR Shift reduction uponsurface mounting technology (SMT) application. The metallic layersignificantly reduces ESR shift by preventing the SMT stresses fromtransferring to the cathode layers. In addition, the coefficient ofthermal expansion (CTE) mismatch significantly decreases due to similarCTE between lead frame and the metal layer.

There is lower edge resistance with very thin silver coating. The thinmetallic layer offers continuous path for current collection even whenthe silver paint is not covered at edges and corners of capacitor.

The metal layer in conjunction with blocking layer provides improvedhumidity performance. When plating alone is used for metallization, freeionics left in the cathode layer can react with certain types of metalplated layers and can increase ESR. The presence of a blocking layer anda metal filled coating inhibit this and offers improved ESR stability.

EXAMPLES Example 1

A series of identical tantalum anodes was prepared. The tantalum wasanodized to form a dielectric on the tantalum anode in identicalfashion. In one set of samples a manganese dioxide cathode was formed onthe dielectric with first carbon layer comprising graphite dispersion inacrylic solution was applied. The capacitors with manganese dioxidecathodes were split into three groups. In a first control group a nickelplated layer was formed on the first carbon. In the second control groupa silver layer was formed on the first carbon. In the inventive group ahydrophobic coating comprising silicone polymer solution was applied onthe first carbon layer. A second carbon layer comprising a mixture ofcarbon black and graphite dispersion in a polyester binder was appliedon the hydrophobic layer. A nickel plated layer was formed on the secondcarbon by electroplating. Both control and hydrophobic layer sampleswere dried and electrical properties were measured. The results arepresented in Table 1.

TABLE 1 Leakage (microamps) ESR (mohms) Plated Layer 536 115 SilverLayer 1.3 57.6 Hydrophobic Layer 1.25 42.8

Table 1 clearly illustrates the advantages of the hydrophobic layer,particularly, with regards to a decrease in leakage current and ESR.

Example 2

On an identical set of samples a polymeric cathode was formed utilizingpolyethylenedioxythiophene (PEDT) with carbon layers applied theretorespectively. The capacitors with PEDT cathodes were split into threegroups. In a control group a nickel plated layer was formed on a firstcarbon layer comprising carbon black and graphite dispersion in apolyester binder solution. In the second control group, a carbon andsilver layer was applied on a PEDT cathode. In the inventive group ahydrophobic coating comprising a silicone polymer solution was appliedon the first carbon layer. A second carbon layer, similar to the secondcarbon layer of Example 1, was applied on the hydrophobic layer. Anickel plated layer was formed on the second carbon by electroplating.Both control and hydrophobic layer samples were dried and electricalproperties were measured. The results are provided in Table 2.

TABLE 2 Leakage (microamp) ESR (mohms) Plated Layer 312.5 41.93 SilverLayer 3.01 47.6 Hydrophobic Layer 0.95 66.5

Table 2 clearly illustrates the advantages offered by the hydrophobiclayer, particularly, with regards to leakage current.

Example 3

On an identical set of samples a polymeric cathode was formed utilizingpolyethylenedioxythiophene (PEDT) polymers. The capacitors with PEDTcathodes were split into three groups. In the first control group acarbon layer was applied on PEDT followed with Nickel plating. In asecond control group a carbon and silver layer was applied on the PEDTcathode. In the inventive group, a hydrophobic layer comprising siliconepolymer solution was applied on the PEDT cathode. No carbon layer wasapplied in the group comprising the hydrophobic layer. A nickel platedlayer was formed on the hydrophobic layer by electroplating.

Both control and hydrophobic layer samples were dried and electricalproperties were measured. The results are provided in Table 3.

TABLE 3 Leakage (microamps) ESR (mohms) Plated Layer 85.44 18.98 SilverLayer 5.108 22.8 Hydrophobic Layer 2.67 19.3

Table 3 clearly illustrates the advantages offered by the hydrophobiclayer, particularly, with regards to leakage current and ESR.

Example 4

Two low ESR capacitor part types of 12 mohm and 9 mohm were chosen forthis study. On an identical set of samples a polymeric cathode wasformed utilizing polyethylenedioxythiophene (PEDT) polymers. Thecapacitors with PEDT cathodes were split into two groups. In the firstcontrol group a carbon layer was applied on PEDT followed by silverlayer application. In the inventive group, a carbon layer as blockinglayer and a silver layer was applied onto the cathode layer followed byelectroplating nickel onto the silver layer. It can be seen that a lowerESR after SMT was obtained in the inventive group. The results arepresented in Table 4.

TABLE 4 Control (mohm) Inventive (mohm) 12 mohm group ESR afterencapsulation 10.77 10.85 ESR after SMT pass 1 13.83 12.03 ESR after SMTpass 2 14.4 12.29 ESR shift 3.63 1.44 8 mohm group ESR afterencapsulation 9.3 8.06 ESR after SMT pass 1 12.38 9.02 ESR after SMTpass 2 12.71 9.44 ESR shift 3.41 1.38

Example 5

A 12 mohm ESR part type was chosen for this humidity exposure study. Onan identical set of samples a polymeric cathode was formed utilizingpolyethylenedioxythiophene (PEDT) polymers. The capacitors with PEDTcathodes were split into two groups. In the first control group a carbonlayer was applied on PEDT followed by a Nickel plated layer. In theinventive group, a carbon layer as the blocking layer and a silver layerwas applied onto the cathode layer followed by nickel plating onto thesilver layer. Both control and inventive parts were subjected to longterm humidity test at 60° C., 90% relative humidity for 1000 hrs. Theresults are presented in Table 5.

TABLE 5 Control (mohm) Inventive (mohm) ESR before humidity test 16.1112.78 ESR after humidity test 32.74 16.97 ESR shift 16.63 4.19

As indicated in the results the combination of a blocking layer and anickel plated layer provides a capacitor with an ESR of less than 20mohm and more preferably less than 15 mohm.

The invention has been described with particular emphasis on thepreferred embodiments. One of skill in the art would realize additionalembodiments, alterations, and advances which, though not enumerated, arewithin the invention as set forth more specifically in the claimsappended hereto.

Claimed is:
 1. A solid electrolytic capacitor comprising: an anode; adielectric on said anode; a cathode on said dielectric; a plated metallayer; a blocking layer between said cathode and said plated metallayer; and further comprising a metal filled layer between said cathodeand said plated metal layer.
 2. The solid electrolytic capacitor ofclaim 1 wherein said metal filled layer is between said cathode and saidblocking layer.
 3. The solid electrolytic capacitor of claim 1 whereinsaid metal filled layer comprises silver.
 4. The solid electrolyticcapacitor of claim 1 wherein said plated metal layer is a plated nickellayer.
 5. The solid electrolytic capacitor of claim 4 wherein saidplated metal layer consist essentially of nickel.
 6. The solidelectrolytic capacitor of claim 1 wherein said plated metal layer has athickness of 2 microns to 100 microns.
 7. The solid electrolyticcapacitor of claim 1 wherein said blocking layer is selected from agroup consisting of a hydrophobic layer, an insulative layer and a layercomprising carbon in a crosslinked matrix.
 8. The solid electrolyticcapacitor of claim 7 wherein said hydrophobic layer further comprises atleast one of a polyhedral oligomeric silesquioxane, silica and nanoclaycoated with hydrophobic polymers.
 9. The solid electrolytic capacitor ofclaim 7 wherein said hydrophobic layer comprises a crosslinked polymer.10. The solid electrolytic capacitor of claim 9 wherein said hydrophobiclayer comprises conductive carbon.
 11. The solid electrolytic capacitorof claim 7 wherein said insulative coating comprises at least one of ahydrophobic polymer and a hydrophobic additive.
 12. The solidelectrolytic capacitor of claim 7 wherein said insulative coatingcomprises a thermoset polymer.
 13. The solid electrolytic capacitor ofclaim 1 further comprising a first conductive carbon layer.
 14. Thesolid electrolytic capacitor of claim 13 further comprising a secondconductive layer.
 15. The solid electrolytic capacitor of claim 14wherein said second conductive layer is a second conductive carbonlayer.
 16. The solid electrolytic capacitor of claim 15 wherein at leastone of said first conductive carbon layer and said second conductivecarbon layer comprises carbon particles selected from the groupconsisting of graphite, carbon black, carbon nanotubes and graphene. 17.The solid electrolytic capacitor of claim 14 wherein said blocking layeris between said first conductive carbon layer and said second conductivelayer.
 18. The solid electrolytic capacitor of claim 14 wherein saidsecond conductive layer further comprises metal particles.
 19. The solidelectrolytic capacitor of claim 18 wherein said metal particles aresilver particles.
 20. The solid electrolytic capacitor of claim 18wherein said second conductive layer comprises at least 5% resin by dryweight.
 21. The solid electrolytic capacitor of claim 18 wherein saidsecond conductive layer comprises at least 20% resin by dry weight. 22.The solid electrolytic capacitor of claim 14 wherein said blocking layeris between said first conductive carbon layer and a metal filled layer.23. The solid electrolytic capacitor of claim 14 wherein said blockinglayer is between a first metal filled layer and a second metal filledlayer.
 24. The solid electrolytic capacitor of claim 1 wherein saidblocking layer comprises a polymer.
 25. The solid electrolytic capacitorof claim 24 wherein said polymer is a polymer of at least one monomerselected from fluorinated monomer and silicone monomer.
 26. The solidelectrolytic capacitor of claim 1 wherein said blocking layer preventsdiffusion of electrolyte into said anode.
 27. The solid electrolyticcapacitor of claim 1 wherein said cathode comprises at least one of MnO₂or a conducting polymer.
 28. The solid electrolytic capacitor of claim 1wherein said blocking layer encases said cathode and said dielectric.29. The solid electrolytic capacitor of claim 28 wherein said blockinglayer encases a portion of an anode wire.
 30. The solid electrolyticcapacitor of claim 1 wherein said blocking layer is less than twomicrons thick.
 31. The solid electrolytic capacitor of claim 1 furthercomprising a cathode lead in electrical contact with said plated metallayer.
 32. The solid electrolytic capacitor of claim 1 with an ESR ofless than 20 mohm.
 33. A solid electrolytic capacitor comprising: ananode; a dielectric on said anode; a conductive polymeric cathode onsaid dielectric; a plated metal layer; a blocking layer between saidconductive polymeric cathode and said plated metal layer wherein saidblocking layer comprises carbon in a crosslinked matrix; and furthercomprising a metal filled layer between said cathode and said platedmetal layer.
 34. The solid electrolytic capacitor of claim 33 whereinsaid metal filled layer is between said cathode and said blocking layer.35. The solid electrolytic capacitor of claim 33 wherein said metalfilled layer comprises silver.
 36. The solid electrolytic capacitor ofclaim 33 wherein said plated metal layer is a plated nickel layer. 37.The solid electrolytic capacitor of claim 36 wherein said plated metallayer consist essentially of nickel.
 38. The solid electrolyticcapacitor of claim 33 wherein said plated metal layer has a thickness of2 microns to 100 microns.
 39. The solid electrolytic capacitor of claim33 wherein said blocking layer is selected from a group consisting of ahydrophobic layer, an insulative layer and a layer comprising carbon ina crosslinked matrix.
 40. The solid electrolytic capacitor of claim 39wherein said hydrophobic layer further comprises at least one of apolyhedral oligomeric silesquioxane, silica and nanoclay coated withhydrophobic polymers.
 41. The solid electrolytic capacitor of claim 40wherein said hydrophobic layer comprises a crosslinked polymer.
 42. Thesolid electrolytic capacitor of claim 41 wherein said hydrophobic layercomprises conductive carbon.
 43. The solid electrolytic capacitor ofclaim 40 wherein said insulative coating comprises at least one of ahydrophobic polymer and a hydrophobic additive.
 44. The solidelectrolytic capacitor of claim 40 wherein said insulative coatingcomprises a thermoset polymer.
 45. The solid electrolytic capacitor ofclaim 33 further comprising a first conductive carbon layer.
 46. Thesolid electrolytic capacitor of claim 45 further comprising a secondconductive layer.
 47. The solid electrolytic capacitor of claim 46wherein said second conductive layer is a second conductive carbonlayer.
 48. The solid electrolytic capacitor of claim 47 wherein at leastone of said first conductive carbon layer and said second conductivecarbon layer comprises carbon particles selected from the groupconsisting of graphite, carbon black, carbon nanotubes and graphene. 49.The solid electrolytic capacitor of claim 46 wherein said blocking layeris between said first conductive carbon layer and said second conductivelayer.
 50. The solid electrolytic capacitor of claim 46 wherein saidsecond conductive layer further comprises metal particles.
 51. The solidelectrolytic capacitor of claim 50 wherein said metal particles aresilver particles.
 52. The solid electrolytic capacitor of claim 50wherein said second conductive layer comprises at least 5% resin by dryweight.
 53. The solid electrolytic capacitor of claim 50 wherein saidsecond conductive layer comprises at least 20% resin by dry weight. 54.The solid electrolytic capacitor of claim 46 wherein said blocking layeris between said first conductive carbon layer and a metal filled layer.55. The solid electrolytic capacitor of claim 46 wherein said blockinglayer is between a first metal filled layer and a second metal filledlayer.
 56. The solid electrolytic capacitor of claim 33 wherein saidblocking layer comprises a polymer.
 57. The solid electrolytic capacitorof claim 56 wherein said polymer is a polymer of at least one monomerselected from fluorinated monomer and silicone monomer.
 58. The solidelectrolytic capacitor of claim 33 wherein said blocking layer preventsdiffusion of electrolyte into said anode.
 59. The solid electrolyticcapacitor of claim 33 wherein said blocking layer encases said cathodeand said dielectric.
 60. The solid electrolytic capacitor of claim 59wherein said blocking layer encases a portion of an anode wire.
 61. Thesolid electrolytic capacitor of claim 33 wherein said blocking layer isless than two microns thick.
 62. The solid electrolytic capacitor ofclaim 33 further comprising a cathode lead in electrical contact withsaid plated metal layer.
 63. The solid electrolytic capacitor of claim33 with an ESR of less than 20 mohm.